1. Field of the Invention
The present invention relates generally to a hydraulic shock absorber, suitable for use in an automotive suspension system. More specifically, the invention relates to a shock absorber having improved piston stroke speed dependent damping characteristics.
2. Description of the Background Art
In general, a hydraulic shock absorber generates a damping force determined by pressure difference across a flow restriction valve structure. As will be appreciated, the pressure difference is variable depending upon the magnitude of the flow restriction at the flow restriction valve structure and the working fluid flow rate. Working fluid flow rate is determined by both magnitude and speed of piston stroke.
When the shock absorber employing a constant orifice is used, the damping force varies at a rate substantially proportional to the square of the piston stroke speed. Therefore, the damping force tends to become insufficient at a relatively low piston stroke speed range so as not to generate a sufficient damping force for successfully damping relative displacement of a vehicular body and a road wheel.
In order to improve this, a two stage disc valve strategy has been proposed for generating a damping force for a relatively low speed piston stroke by a first stage valve and for a higher speed piston stroke by a second stage valve. Such a two stage disc valve strategy has been proposed in German Patent 833 574, for example. The proposed shock absorber has the first stage and second stage disc valves arranged in a tandem fashion. The first stage disc valve is principally active for generating a damping force at a relatively low piston stroke speed range. On the other hand, the second stage disc valve is principally active for generating a damping force at a higher piston stroke speed range. Therefore, combining the first and second stage disc valves, improved piston stroke speed dependent damping characteristics can be obtained. Namely, in the aforementioned German Patent, the damping force varies at a rate substantially proportional to a two-thirds (2/3) power of the piston stroke speed.
On the other hand, in view of the ease of tuning of an automotive suspension system for achieving both vehicular riding comfort and driving stability, it is desirable to provide a shock absorber having damping characteristics linearly proportional to the piston stroke speed. In view of this requirement, the conventionally proposed shock absorbers are not satisfactory.